Method for forming screw threads



March 16, 1965 G. o. CONNER 3,173,282

METHOD FOR FORMING SCREW THREADS Filed March 25, 1965 3 Sheets-Sheet 2 WALL 0F COUPL ING E007' 0F THREAD United States Patent 3,173,282 Patented Mar. 16, 1965 dee 3,173,232 METHD FOR FORMING SCREW THREADS Guy 0. Conner, Cleveland, Ohio, assigner to Republic teel Corporation, Cleveland, hio, a corporation of New Jersey Filed Mar. 2S, 1963, Ser. No. 267,543 6 Claims. (Si. 72-91) This invention relates to improvements in screw threaded objects and more particularly to a method for forming threads in or on an object. T his application is a continuation-in-part of my now abandoned application for United States patent, Serial Number 796,357, filed March 2, 1959, for Method for Forming Screw Threads; said latter application in turn being a continuation-inpart of my now abandoned application for United States patent, Serial Number 465,878, tiled November l, 1954, for Forged Threaded Article.

Heretofore, the most common method of forming threads on or in an article has been by the use of dies or taps. Sometimes the threads have been cast and sometimes they have been rolled. Where the greatest accuracy has been desired, they have been ground. Furthermore, crude threads have been rolled, this latter method being .commonly used in the formation of threads in container caps of thin metal.

The foregoing practices have not always resulted in a desired product, particularly where the threads are internal in character.

Among the objects to which the invention is especially applicable are pipe couplings. These couplings, as is well known to those versed in the art, are used for connecting sections of pipe together end-to-end. The most common practice followed in threading these couplings is to pass a ytap completely through the coupling from end-to-end.

This is especially true of threaded couplings formed in the smaller sizes of pipe.

As can be appreciated such a coupling lacks a very yimportant attribute, namely, that of a tapered threaded internal surface. Since they are used in conjunction with the end of a pipe which has a tapered thread, it can be yseen that at the most, the joint realized is only a compromise from that which could be obtained if both the coupling and the pipe had complementary tapered surfaces.

Another disadvantage of all threaded devices in which the threads are cut with a tap or rolled or ground resides in the resultant thinness of the last thread at the exit to the opening. This threading tapers from a full profile to a paper thinness which leaves a portionextending from the very end inward a considerable distance that is very fragile and which may be easily bent. Since the threading operation has an additional tendency to harden the metal, this thin iin of material frequently breaks olf during the act of assembling the male threaded member in the female threaded member, obstructing the passage and causing pieces of broken thread metal to fall between the engaging threads with the result that the joint is mutilated. Because the threads in the female member, measuring from crest to crest, are of constant diameter throughout the length of the coupling, it is also difficult to start the assembly operation without misalignment of the two threaded members with the result that the threads are torn and the sealing properties of the joint are destroyed. It will be appreciated that the end thread, because of its fragility, has little or no holding power and, as stated above, actually acts as a deterrent to good assembly practices.

Rolled threads have heretofore lacked uniformity in height from base to crest, lengths of the threads being truncated by failure of the thread metal to iill completely the threads of the forming tool.

Another thing to be observed, particularly in connection with the manufacture of threaded nuts, is the character of the material used for the raw stock. lt is common practice to utilize the crop end of the steel ingots in making nuts. rPhe crop end of ingot is that part which is not suitable for other purposes because it includes therein voids due to the rising gases in the ingots, slag, and other impurities, all of which rise toward the top during the pouring and cooling process. The result is that when nuts made from this material are tapped by the normal tapping process, these voids are enlarged such that an undesirable rough thread is produced.

The thread of the present invention may be made with a smooth, highly nished surface free from undesirable voids and without hns. rthese threads can also be made extremely accurate. Furthermore, articles formed according to the method herein may be threaded rapidly and without loss of material due to the cutting away of the material as is common in prior practices.

l have chosen to illustrate my invention in connection with the formation of a pipe coupling having threads that taper from each end toward the middle. This is a threaded structure that would be particularly ditlicult to make if taps were to be used because it would require at least two operations to thread each coupling. My thread forming method involves a forging operation wherein a rotating anvil is used and wherein the workpiece and the anvil are rotated at the same speed. A suitable apparatus for performing the method is illustrated in my United States Patent 2,932,222, issued April l2, 1960.

As illustrated herein, both threads and the chamfers at the ends of the threads are formed simultaneously. It will be appreciated, however, that in its broader aspects,

lthe method is applicable to the formation of other threaded devices and it is not my desire to limit the invention specifically in its application to the article described and illustrated which is used herein merely for the pur.- pose of explaining the invention.

In the drawings:

FIG. l is a vertical medial section of a pipe coupling having the improved thread of my invention;

FlG, 2 is an end elevational view thereof;

FIG. 3 is a developed end portion ofthe thread;

FIGS. 3a, l3b and 3c are sectional views taken on lines ,a-a, b-b and c-c respectively of FIG. 3;

FIG. 4 is a View similar and corresponding to that of FIG. 3 of another embodiment of the invention;

tFIGS. 4a, 4b and 4c are sectional views taken on lines a-a, b-b and c-c respectively of FIG, 4;

FIG. 5 is a fragmentary View of a male thread member;

FIG. 6 is an elevational view of an anvil used in the threading operation;

FIG. 7 is a diagram explanatory of metal flow during a thread forging operation;

PIG. 8 is' asomewhatenlarged section of FIG. 7 showing conditions resulting in an unsatisfactory flow of metal;

FIG, 9 is a somewhat enlarged section of FIG. 7 showing conditions resulting in a satisfactory flow of metal; and

FIGS. 10a, 10b, 10c and 10d schematically illustrate the principal steps of the thread forming operation,

Throughout the drawings like parts have been designated by like reference characters.

Briefly, my invention comprises a method for forming a threaded structure whereof the threads are formed by a forging operation such that the metal is displaced from the body of the article being threaded. The terminal part of the thread has a desirable prole to its very end where it merges with the wall of the article and is free from tins or thin sections. As will appear later, the degree of the merging of the thread with the wall is important from the standpoint of manufacture as well as its operation when the device is assembled with other parts.

In FIG. 1, I have shown a pipe coupling wherein each end is threaded. The threads from Opposite ends are tapered toward the middle; that is, the internal diameter of the threads measuring from the crest of one land t0 the crest of the diametrically (except having pitch) opposite land gradually decreases toward the longitudinal center of the coupling.

In this instance the wall of the coupling is indicated by the reference charatcer 10, and although the exterior may be substantially cylindrical, I prefer to make it tapered inasmuch as this enables the pieces to be chucked readily for the threading operation in a chuck having a tapered socket. Although the taper may vary, a preferred degree of taper would be that of 3.

As can be seen best in FIG. 1, two distinct threaded parts are provided at 14 and 15, each being a preferred form of pipe taper. The threaded roots 11 are disposed in a cone which tapers inwardly toward the middle of the coupling. The crests 12 are likewise disposed in a similar cone, except for the extremities about which more will be said later.

At the eXtreme ends there are provided zones 16 which lie in an extension of the cone of the threaded roots. Although the threads at the center portion may meet, a desirable formation is to have them terminate short of the center, leaving a Zone 17 which is also an extension of the cone of the thread roots. Thus, it will be seen that the threads extend above the base of the body as distinguished from ordinary cut threads, where the threads are formed by gouging out the material of the body. Since the method by which the threads are formed involves a forging operation, there is no waste material. No chips drop into the threads to mutilate the same and the surface of the thread presents a smooth, highly polished accurate surface.

Of particular importance is the formation of the eX- tremities of the thread. maintaining the thread contour or proliie throughout the extremity of the thread, which extremity decreases sharply in height to the point where the crest merges with the root cone.

This formation is best shown in FIG. 3, which is a developed view of the thread from a full thread to its end, together with the sections of FIGS. 3a, 3b and 3c. AS can be seen in FIG. 3a, the full thread is represented by a triangle of 60. The succeeding iigures show that as the thread nears its end, it maintains the same profile down to its very end where it disappears into the zone 16 which replaces the usual chamfer. As can be seen best in FIG. 2, this results in a convex or reversely curved crest at 18, as viewed from the threaded face.

FIGS. 4, 4a, 4b and 4c show a similar thread terminal except that the triangular formation of the full thread iS truncated throughout the terminal portion of decreasing height.

It should be pointed out that instead of being convexly curved, the terminal portion of the thread can be formed along a line extending tangent from the crest of the last full thread and meeting with the wall of the threaded article at an obtuse angle, as shown at 19 in FIG. 2.

The application of the invention to a male threaded member is illustrated in PIG. 5. This also may be formed Basically, this contemplates by a forging operation but in this instance, as will be apparent, the forging die must be a female member whereas in the formation of internal threads, the die must be a male member. In this case the metal at the shank 30 is substantially in continuation of the pitch line of the threads. This is due to the fact that the forging operation causes a movement of the metal from the roots 31 upwardly into the adjacent crest 32. The terminal end of the thread, however, as indicated at 34, merges vith the end 33 in a mannerrsubstantially as described for the previous embodiment. The terminal end 33 forms a short cylindrical portion, the surface of which lies in the same cylinder as the roots of the threads. It will be apparent that the same type of male thread may be provided on the end of a pipe except for the fact that it would be the reverse of that shown in the coupling.

As previously mentioned, the manner in which the terminal portion of the thread is formed accounts for several important aspects of the improved product. It avoids the thin sections previously discussed, providing a thread having maximum strength down to its very end and thereby reducing the chances of the thread being mutilated and, as previously stated, it facilitates the assembly of threaded articles. It completely eliminates the thin frangible sections which are present in most threaded articles.

Basically, the method herein envisions the controlled flow of the thread metal by use of a mandrel or anvil which has a properly formed surface that may be engaged with the surface of the article being threaded and pressed firmly together and rotated at the same r.p.m. which causes a controlled flow of the metal from the article into the matrix of the anvil.

It is important to note that the threaded length of the mandral is so related to the length of the workpiece that the thread termini of the mandrel are maintained within the contines of the workpieces throughout the threading operation, i.e., they are located inwardly of the respective ends of the workpiece. If the terminals of the forming threads of the mandrel are formed to merge with the mandrel wall at a fairly sharp angle, an effective barrier is formed to the uncontrolled tlow of thread metal along the length of the thread in the direction of the thread terminus when the mandrel is operating in contact with the workpiece. When the thread metal is substantially prevented from flowing toward the terminal end of the thread as the thread is being formed, a full thread body will be formed by eX- trusion of more of the thread metal in a radial direction. Thus, full, sharp threads having the desired cross-section along their entire length are obtained, and the thin, fragile, tapered terminal ends are avoided.

Such a mandrel, as illustrated in FIG. 6, includes a shank 4t) which may be gripped in a suitable driving mechanism, not shown. The operating parts of the anvil include the tapered thread forming surfaces 41 and 42, which will form the threads 14 and 15 of FIG. 1, for example. Zones 16 of the article in FIG. 1 are formed by the portions 44 at the opposite ends of the threads 41 and 42 and zone 17 of the article is formed by the part 45 intermediate the two threads of the mandrel. A portion 36 of the chuck which supports the article is shown in FIG. 1.

It should be borne in mind that since, in this instance, the work surface is moving in contact with the anvil at a greater linear Speed than the forming surface of the anvil, the surface of the anvil actually comprises a compressed version of the final thread which it forms. However, the proiile of the thread formed in the article is defined by the profile of the lands and grooves on the anvil which, in the case of a sharp V thread are 60. and extend around the anvil in a spiral manner which determines the pitch of the thread to be formed. The form of the terminal end of anvil thread, as stated, is of particular importance. Unless the terminal end is properly designed the metal of the article being threaded will flow into and block the groove in the anvil, and after the thread has been partially formed, the usefulness of the anvil will be destroyed since the lands of the anvil threads near the terminal will be broken by the crowding of the material into this terminal. Therefore, I have found that in the formation of the anvil, if the profile is maintained and the crest of the thread is brought out to the root line at a sharp inclination, that the metal of the article being threaded can be largely restrained from ow beyond the terminus of the groove in the anvil and can be caused to ow into the wall of the article, and more particularly into the thread being formed, where it merges smoothly with the article wall and its thread.

It appears that a very desirable circumferential contour for the thread terminal of the anvil is formed by grinding it with a V-shaped wheel having a substantially larger radius than the anvil thread which would provide the form of thread shown at 18 in FIG. 2. If the radius of the grinding wheel is large enough, all that needs to be done is to stop rotation of the anvil at the terminal thread and withdraw the wheel from contact therewith. However, a smaller wheel can be used provided that when the terminal'is reached, the wheel be drawn from the work along a line that is tangent to the circumference of the groove that has just been cut. The effect of this is shown at 19 in FlG. 2. l The successful performance of the method of the invention is dependent in large measure on the manner in which the forming tool and the workpiece are manipulated in respect to each other. A fundamental requirement of the method is that the forming tool and the workpiece be rotated in contact with each other at the same angular velocity, and that the tool and the workpiece both be positively driven. Furthermore, it is also important that the relative movement of the workpiece and the tool, as these two members are brought into engagement with each other and later withdrawn from engagement with each other, be along a straight, radial path that passes through the center of both the workpiece and the tool.

a straight line AC, determines whether or not the material can flow beyond the end of the corresponding groove in the forming die or anvil. For instance, if the curvature of the crest at this point were on a radius, r, FIG. 8, equal to the height, h, of the thread, FIG. 8, a chord AD drawn from the point A where the crest leaves the circumference, to point D where said crest meets the circumference indicated by the cone of the roots of the thread, would provide an included angle BAD of 45 with a line drawn through the radius of the article (the baseline) and intersecting the point A where the crest of the thread leaves the circumference. Since the material is being moved circumferentially (it is known that extrusion occurs in the direction of the thread as the thread is formed), it would strike the end of the groove at this point in a manner which, at the midpoint, would cause deflection of the metal at 90; if the crest were curved on a radius equal to the height of the thread, h, the metal would tend to be deflected backward through an angle of 180 at the root of the thread. However, if the mergence of the crest with the outer wall is provided by a groove in the forming die or anvil, the radius, r, of which is such that a chord AC, FG. 9, drawn in the manner indicated would As to the tool employed, the important thing to be borne in mind is that the conliguration of the terminus of the thread forming groove of the die or anvilvrnust be such that the material of the workpiece can not dow freely or in any substantial amount beyond the end of the groove of the mandrel. To this end the inclination of the terminal part of the forming thread is calculated as to form an effective barrier against iiow of thread metal in the direction of the thread. This inclination will vary with the strength of the material of which the tool is made and the hardness of the workpiece in which the thread is being formed. Whatever these variables, the proper inclination of the terminal thread of the forming tool can be derived by one skilled in the art in View of the teaching to follow.

insofar as the threading phenomenon is presentlyY understood, it appears that during the forming operation, the metal is extruded along the direction of the thread being formed and that the terminus of said thread is displaced into the wall portion of the article. It is important that the end of the thread being formed merges into the wall of the article Without shearing or forming a plane ofweakness.

It appears that the practical limits of this merging of the crest with the wall are determined by the included angle of a triangle, the base line of which intersects a point A, FIG. 7, where the terminal thread diverges from the circumference of the crest, said base line extending from said point A along the radius of the coupling and projecting along said radius to a point B at the root of the thread; the other corner of the triangle being the point C where the crest merges with the wall. The inclination of the crest along the terminus of the thread relative to this base line, whether said crest terminus be a curved or make an angle BAC with the base line greater than 45, then the material can be made to llow into the wall of the groove. This angle, for most applications, need not be substantially greater than 45 to secure a proper control of thread metal ilow. Indeed, the smaller the angle consistent with the strength of the tool when considered in light of lthe hardness of the workpiece, the better Will be the results. Obviously, the base angle must not be so small as to produce such resistance to tool and workpiece rotation which would damage either tool or workpiece.

lt is peculiar to the formation of such an article that the extremity of the thread has a tendency to lill up that portion of the groove of the mandrel which forms the extremity of the thread and which, without my invention, would cause a blocking of the formation and a destruction of the anvil at this point. By providing the circumferential contour of the thread terminus, as shown and described, the metal flows into the wall of the workpiece smoothly and without hindrance where the crest of the mandrel thread merges with the wall and is practically invisible. It is apparent, therefore, that the configuration of the thread terminus on the forming tool, as shown in FG. 6, must be such as to permit the metal to flow into the die easily during the progressive formation of the thread. lt must be a primary objective to avoid excessive extrusion of the metal at the ends of the thread which would otherwise result in a seam or a burr. On the other hand, the metal must be controlled to flow in such manner as to completely form the desired thread shape at the leading and trailing ends of the thread.

As will be apparent, such a mandrel or anvil is of a considerably smaller diameter than the internal diameter of the coupling where internal threads are being made with a result that, although the mandrel and workpiece Vare being rotated the same number of revolutions, the

peripheral speed of the two contacting parts is different. Assuming a tool, shaped as shown in FlG. 5, is used to form a thread in a coupling, as illustrated in FIG. 1, it will be observed that the maximum diameter of the tool (thread crest to thread crest on opposite sides of the tool) must be somewhat less than the crest diameter of the formed thread in the coupling, i.e., the distance from crest to crest on opposite endof a diameter of the coupling.

ln the case of internal threading, the surface of the workpiece will be moving ahead of the mandrel and in the case of the external threads, the surface of the anvil will be moving ahead of the workpiece. In either event, there is a sliding action apparent which supplements the flow of the metal from the root to the crest which provides the smooth, polished surface tending to close the voids, preventing the formation of cracks, and eliminating undesirable tins or burrs.

This phenomenon can be demonstrated by resort to an example. Although the primary illustrations involve the forming of a bi-conical thread, the followingy discussion is equally applicable to the formation of a cylindrical thread or a thread of any other configuration. lt has been found in this work that in forming threads, wherein the cross-sectional shape thereof is such that the thread has a sharp crest as well as a sharp root form, that in general the heighth of the metal displaced from the blank to form the crest of the thread is approximately equivalent to the depth of the thread that results from metal displaced out of the blank. It will be noted that the area of the crosssection that forms the root of the thread is of greater Volume than the volume of revolution that includes the crest of the thread, since the diameter of the root is greater than the diameter of the crest; accordingly, there is more material displaced into the crest of the thread than is taken from the root. In other words, the mean diameter of worked material lies closer to the root of the thread than it does to the crest of the thread. Let it be assumed that a thread having a nal diameter of l inch is to be formed in a coupling. In orderito do this, the inside thread diameter, i.e., crest to crest across the diameter, will be 7A; of an inch in the final product; the root diameter, i.e., the distance across the diameter from the root of one thread to the root of a thread opposite, will be 1 inch. ln order to form this type of thread, Ordinarily a 1%@ inch diameter tool will be employed; that is, the diameter of the tool from the crest of the thread on one end of the diameter to that of the crest on the opposite end of the diameter will be 1%6 of an inch. The mandrel will have formed therein Ithe forming threads, as described;

the terminals thereof having a shape such as that imparted by a 10 inch grinding wheel separated from contact with the tool in a radial direction at the thread terminals.

When this tool is first brought into contact with the work, which ordinarily would have an inside diameter of 1%6 inch, the lineal speed of the tool at the point of contact will be 100g/,6 inch or .255 of an inch per minute the r.p.rn. The surface speed of the inside of the blank at the point of contact with the ltool will be 2.94 inchesXthe rpm. or there will be a slippage between the tool and the work of approximately .45 of an inch the r.p.m. At the end of the forming operation, when the tool has progressed into the work to its full limit, the crest of the threads on the forming tool will be in engagement with the root of the threads of the formed coupling, and since these two elements have the diameters respectively of 13A@ inch and l inch, the respective surface speeds thereof are 2.55 Xthe rpm. and 3.l4 the rpm.,V

or a difference of approximately .59 inch the rpm. It

tion progresses, but that the respective loci of points on the surface of the tool do not change with respect to the corresponding points on the work, and at each revolution of the tool and work, these respective loci come into engagement and displace metal from the blank in a Vradial direction along the thread forms of the tool.

The foregoing procedure is schematically illustrated in FIGS. 10a, 10b, 10c and 10d. These figures are drawn to approximately twice the scale utilized in the foregoing example. In FIG. 10a, the thread d?. of the anvil is shown concentrically located within the workpiece 1.0; this being the initial position of the pieces as the operation is initiated. The workpiece 10 is now rotated about its axis `at a pre-selected and constant angular velocity and the forming tool having the threads i2 thereon is also rotated about its axis at the same angular Velocity and direction as that of the workpiece. FiG. 10b shows that the workpiece 10 and the forming tool having the threads 42 thereon are moved relative to each other along a path lying in the plane .that includes the axes of both members while maintaining the axes parallel to each other.

FIG. 10c Shows the stage in which the rotation and translation of the two members is continued while applying force so that the thread 42 of the tool is impressed into the cylindrical surface of the workpiece whereby the metal in the surface of the workpiece and adjacent thereto is forged into the configuration on the forming tool to completely ll the pattern on the face of the tool to conform exactly thereto while maintaining the two members in rotation at the same pre-selected angular velocity. Finally, FIG. 10d indicates the withdrawal of the parts from mating engagement along the path lying in the plane that includes the axes of both members to clear the tool of the workpiece.

The working of the metal in this manner also removes irregularities in wall thickness. For instance, should the bore of a blank being threaded be eccentric to the exterior, the working of the article during threading will cause a displacement of the metal from one side to the other providing a resultant product having an even wall thickness all the way around.

When the teaching herein is put to practical use, it is applied in full consideration of the strength of the forming tool and the hardness of the workpiece on which the tool is intended to operate.

This observation is particularly true in determining the radius of the groove in the forming die at the thread terminal. It has been indicated above that merger of the thread crest with the wall of the workpiece is determined by the included angle of a triangle whose base line is a radius of the coupling and whose side is the line along which the crest of the thread merges with the wall of the coupling. It is obvious that the inclination of the crest of the thread along the terminus thereof relative to the radius of the workpiece can be very sharp, i.e., in the very close neighborhood of 45, if the tool is strong and the workpiece is relatively soft. On the other hand, this inclination Vmust be formed at a greater angle such that the thread inclination at its terminus is more gradual if the metal of .the workpiece is relatively hard.

Suitable steels for the construction of the forming tool are the following:

Republic CV [A.I.S.I. No. Sl] C 0.50 Cr 1.20 W 2.20 V 0.20 Mn 0.30 Si 0.25 Fe Balance Republic l2 HW [A.I.S.I. No. Modified H12] Crucible Rex 4V [A.I.S.I. No. T9]

C 1.25 Mn 0.30 Si 0.30 Cr 4.00 V 4.00 W 18.50 Mo 0.75 Fe Balance Of the foregoing steels, Republic CV and Republic l2 HW have both been used with satisfactory results. Of the two, Republic 12 HW appeared to be the more suitable.

A.I.S.I. C i M11 I I (Max.) 5 S (Max.)

0. (iS/0. 13 0. 30/0. 60 0. 040 0. 050 O. 18/0. 23 O. 30/0. 60 0. 040 0. 050 0. 43/0. 50 O. 60/0. Q0 0. 040 0. 050

Although l have described my invention in conjunction with the formation of a sharp V-shaped thread, it will be apparent to those versed in the art, and it is so my desire, that it should apply also to the formation of threads having other proles.

Having thus described my invention, I am aware that numerous and extensive departures may be made therefrom without departing from the spirit or scope of the invention as defined in the appended claims.

What is claimed is:

l. The method of forming screw threads which comprises the steps of extruding metal from the workpiece in a radial direction, substantially conning the terminus f the thread metal against longitudinal ow in the direction of the thread being formed, displacing the terminus of the thread metal into the metal of the workpiece, and controlling the ow of the thread metal so that the inclination of the crest of the thread along the terminus thereof relative to the radius of the workpiece is not substantially more than 45 and consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

2. The method of forming screw threads which cornprises the steps of extruding metal from the workpiece in a radial direction, substantially confining the terminus of the thread metal against longitudinal ow in the direction of the thread being formed, displacing the terminus of the thread metal into the metal of the workpiece by controlling the tow of the thread metal so that the inclination of the crest of the thread along the terminus thereof relative to the radius of the workpiece is not substantially more than 45, and maintaining the cross-sectional contour of the thread to the end thereof where it merges with the metal of the workpiece so that consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

3. The method of forming screw threads which comprises the steps of moving a threaded forming tool into Contact with a workpiece along a radius of the workpiece, imparting rotation to both the forming tool and the workpiece whereby both are driven at the same number of revolutions per minute, extruding the metal of the workpiece in a radial direction, substantially confining the terminus of the thread metal against longitudinal flow in the direction of the thread being formed, displacing the terminus of the thread metal into the metal of the workpiece, controlling the flow of the thread metal so that the inclination of the crest of the thread along the terminus thereof relative to the radius of the workpiece is not substantially more than 45, and controlling the flow of the threal metal at the terminus of the thread in said forming tool such that the cross-sectiond contour of the thread is maintained to the end thereof where it merges with the metal of the workpiece and consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

4. The method of forming screw threads which comprises the steps of forcibly moving a threaded forming tool into contact with a workpiece along a radius of the workpiece, imparting rotation to both the forming tool and the workpiece whereby both are driven at the same number of revolutions per minute, by such Contact and rotation extruding the metal of the workpiece in a radial direction, providing a barrier at the thread terminal of the forming tool and maintaining said barrier within the contines of the workpiece throughout the threading operation thereby substantially confining the terminus of the thread metal against longitudinal ilow in the direction of the thread being formed, displacing the terminus of the thread metal into the metal of the workpiece, and controlling the flow of the thread metal at the terminus of the thread in said forming tool such that the cross-sectional contour of the thread is maintained to the end thereof wn re it merges with the metal of the workpiece and consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

5. The method or" forming screw threads which comprises the steps of forcibly moving a threaded forming tool into Contact with a workpiece along a radius of the workpiece, imparting rotation to both the forming tool and the workpiece w iereby both are driven at the same number of revolutions per minute, by such contact and rotation extruding the metal of the workpiece in a radial direction, substantially confining the terminus of the thread metal against longitudinal flow in the direction of the thread being formed, displacing the terminus of the thread metal into the metal of the workpiece, controlling the flow of the thread metal so that the inclination of the crest of the thread along the terminus thereof relative to the radius of the workpiece exceeds 45 only to the extent required by the strength of the forming tool and the hardness of the workpiece, and controlling the llow of the thread metal at the terminus of the thread in said forming tool such that the cross-sectional contour of the thread is maintained to the end thereof where it merges with the metal of the workpiece and consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

6. The method of forming screw threads which comprises the steps of forcibly moving a threaded forming tool into contact with a workpiece valong a radius of the workpiece, imparting rotation to both the forming tool and the workpiece whereby both are driven at the same number of revolutions per minute, by such Contact and rotation extruding the metal of the workpiece in a radial direction, providing a barrier at the thread terminal of the forming tool and maintaining said barrier within the contines of the workpiece throughout the threading operation thereby substantially contining the terminus of the thread metal against longitudinal flow in the direction of the thread bein r formed, displacing the terminus of the thread metal into the metal of the workpiece, controlling the iiow of the thread metal to form an inclination of the crest of the thread along the terminus thereof which is sharp relative to the radius of the workpiece, and controlling the ilow of the thread metal at the terminus of the thread in said forming tool such that the cross-sectional contour of the thread is maintained to the end thereof where it merges with the metal of the workpiece and consecutive sections of the terminal thread have a similar geometrical relationship to the complete thread form.

References Sited in the iile of this patent UNITED STATES PATENTS 329,900 Harvey Nov. 10, 1885 1,946,735 Frayer Feb. 13, 1934 2,251,495 Owen Aug. 5, 1941 FOREIGN PATENTS 906,263 France May 14, 1945 

1. THE METHOD OF FORMING SCREW THREADS WHICH COMPRISES THE STEPS OF EXTRUDING METAL FROM THE WORKPIECE IN A RADIAL DIRECTION, SUBSTANTIALLY CONFINING THE TERMINUS OF THE THREADED METAL AGAINST LONGITUDINAL FLOW IN THE DIRECTION OF THE THREADED BEING FORMED, DISPLACING THE TERMINUS OF THE THREAD METAL INTO THE METAL OF THE WORKPIECE, AND CONTROLLING THE FLOW OF THE THREAD METAL SO THAT THE IN- 